Circuit boards are widely used in the electrical industry for radio, television, computers, appliances, industrial and electronic equipment. Printed circuit boards have been traditionally manufactured from copper clad epoxy-glass laminates.
When starting with this material the shape of the printed circuit board must first be routed out and the holes for mounting the components (e.g., transistors, resistors, integrated circuits, etc.) individually drilled. The board is then masked with photoresist, the circuitry imaged and the copper etched away from areas where it is not wanted.
An alternative to this procedure is to injection mold the circuit board substrate with the holes in place or even with parts such as clips and bosses as part of the molded board. The molded substrate is then put through several adhesion promotion steps and plated with electroless copper according to standard technology, to produce the printed circuit board.
In this case the substrate material is limited to thermoplastic resins with sufficient thermal stability and chemical properties to survive wave soldering. Savings may result with these injection molded circuit board substrates due to the elimination of considerable mechanical processing such as routing and drilling.
The critical parameters of a printed circuit board, from a soldering standpoint, are its heat deflection temperature, environmental stress crack resistance and thermal expansion coefficient. The higher a substrate's heat deflection temperature and environmental stress crack resistance to solder fluxes, the less likely it will blister or delaminate during soldering. Similar advantages of molded substrates in manufacture of other parts with selected portions metalized can be seen, even when their environmental and thermal requirements are different.
Methods for the electrochemical and electroless deposition of copper and other metals onto activated substrates are well known and defined in the art and will not be detailed herein. Methods for manufacturing planar printed circuit boards from metal/resin laminates by subtractive etching of the copper are also well documented in the art.
Recent developments in the art include the use of features in molded circuit boards. One example is a molded-in pattern of recesses which, when the raised portions are coated with a resist or are selectively metal plated, define the circuit traces.
This technology is disclosed in U.S. Pat. Nos. 4,532,152, 4,651,417 and 4,6687,603. Use of these techniques requires a material onto which copper may be plated at some stage after the molding of the part.
A large number of documents in the literature discuss imaging circuit traces by selective deposition of metal particles followed by electroless plating of a metal. These include U.S. Pat. Nos. 3,629,922, 3,772,056, 3,772,078, 3,907,621, 3,930,963, 3,959,547, 3,993,802, 3,994,727, 4,511,597, 4,594,311 and 4,666,739. In these techniques, it is important to adhesion promote the surface of the item to be plated in order to get good mechanical adhesion of the electoless metal deposit.
Other methods for initiating metallization of previously inert plastic substrates can be found in U.S. Pat. Nos. 4,327,124, 4,447,471, 4,478,883, 4,493,861 and 4,554,182. These include activation by special inks, bromine compounds, and ionic polymers. In some cases these require adhesion promotion of the substrate, and/or a chemical attack to modify wettability is needed.
Other examples exist in which the catalyst for electroless plating is incorporated into the material formulation itself, as in U.S. Pat. No. 3,629,185 and 4,281,038. These catalysts include metal particles and metal-coated particles.
The image of the circuit trace can be obtained by various methods known in the art, including use of resists before or after initial cooper plating, or by overmolding with a noncatalytic resin as disclosed in European Patent Application Nos. 0 256 428 and 0 192 233. In these cases, it is necessary to etch the material both to promote good adhesion and to expose the catalytic particles, which are generally remote from the original molded surface of the device.
The demand for nonconductive moldable materials that will accept electroless deposits of copper and other metals is thus clear. In common in all the above techniques of electroless metal deposits is the need to etch, adhesion promote, or otherwise modify the surface of such a molded part.
Many techniques for chemical etching of thermoplastics are disclosed in the literature including U.S. Pat. Nos. 4,457,951, 4,592,852, 4,592,929, 4,595,451, 4,601,783, 4,610,895 and 4,629,636. In general, these techniques require the material to be wet or penetrated by reagents or solvents.
Alternative methods utilizing plasmas, as in U.S. Pat. Nos. 4,337,279 and 4,402,998 are more expensive and difficulties are encountered with uniform etching and penetration into holes. For these reasons chemical etching methods involving solvent penetration into the material are preferred.
Available materials are, however, limited in scope due to this need for chemical attack, which is usually dependent on penetration of a solvent into the material. This is best accomplished with an amorphous polymer, which typically has low solvent resistance, as compared to a semicrystalline polymer. Thus, the most common polymers used in the art for molded circuit boards, polyethersulfone (PES) and polyetherimide (PEI) are amorphous polymers.
There are many advantages to use of semicrystalline polymers. A property of prime importance in the field of molded circuit boards is a high heat deflection temperature (HDT) since in many cases it is desirable to solder electrical components onto these devices, generally at temperatures in the range of 230.degree. to 290.degree. C. HDT is measured via ASTM D648, however, tests for dimensional stability of the molded parts in these temperature ranges are the standard by which sufficiently "high HDT" is judged.
For an amorphous polymer this requires a very high glass transition temperatures (Tg) which restricts the available polymers to an elite group. These polymers are typically expensive, require high processing temperature, and have very poor flow characteristics.
Semicrystalline polymers, on the other hand, can be compounded with glass fibers or other fillers to give an HDT determined more by their melting temperature (Tm) rather than their Tg. Generally, at least about 10% glass fiber is needed. This allows much higher HDT's than would otherwise be indicated by their low melt viscosities and processing temperatures.
This would allow for use of much less expensive polymers with lower processing temperatures and good flow characteristics. For example, poly(cyclohexylenedimethylene terephthalate) (PCT) and crystallizable copolymer thereof, a polyester having repeat units from terephthalic acid and 1,4-cyclohexanedimethanol, can withstand total immersion in a 260.degree. C. solder bath for a full minute with little ill effect while PES, which has similar processing temperature with higher melt viscosity, and is more expensive, has a Tg about 40 degrees below such a 260.degree. C. bath.
Examples of materials in use in the industry are given in Table 1 along with some potentially competitive semicrystalline polymers, poly(ethylene terephthalate) (PET) and crystallizable copolymer thereof, a polyester having repeat units from terephthalic acid and ethylene glycol and PCT. Semicrystalline polymers clearly would be competitive on cost and HDT if they could be made to accept electroless metal plating with good adhesion.
TABLE 1 ______________________________________ Platable High Temperature Thermoplastics Polymer HDT* (.degree.C.) ______________________________________ Polyarylate (PA) 155-179 Polysulfone (PS) 168-181 Polyetherimide (PEI) 197-223 Polyethersulfone (PES) 203-216 GFR** PET 216-243 GFR** PCT &gt;260 ______________________________________ *Range of HDT depends on amount of filler. (Information from various issues of the publication Plastics World.) **Glass Fiber Reinforced (GFR), and in a crystalline state.
U.S. Pat. No. 4,520,067 discloses the use of a polymer blend of two amorphous polymers, PES and PS. While both polymers are useful alone, the blend has better etchability than the PES and higher HDT than the PS. This blending thus achieves properties that are intermediate between the two components but is qualitatively the same as both components of the blend, which are themselves both amorphous polymers that are substantially etchable.
There is, however, still a need for a composition which would utilize semicrystalline polymers in molded articles which would accept electroless plating to thereby derive benefits in terms of physical properties from the semicrystalline polymer while still achieving good copper adhesion as is seen with amorphous polymer.